U.S. patent number 7,888,754 [Application Number 12/317,692] was granted by the patent office on 2011-02-15 for mems transducer.
This patent grant is currently assigned to Yamaha Corporation. Invention is credited to Masayoshi Omura, Tamito Suzuki, Yukitoshi Suzuki.
United States Patent |
7,888,754 |
Omura , et al. |
February 15, 2011 |
MEMS transducer
Abstract
An MEMS transducer is constituted of a diaphragm, a plate, a
support structure for supporting the diaphragm and the plate with a
gap layer surrounded by an interior wall, an electrode film (e.g. a
pad conductive film) for covering a contact hole formed in the
support structure, and a protective film (e.g. a pad protective
film) which is formed on the support structure externally of the
interior wall so as to cover the side surface of the electrode film
having low chemical stability. The protective film is formed in the
limited area including a part of the surface of the electrode film
except for its center portion and the surrounding area of the
electrode film. This allows the protective film to use materials
having high membrane stress such as silicon nitride or silicon
nitride oxide.
Inventors: |
Omura; Masayoshi (Hamamatsu,
JP), Suzuki; Tamito (Fukuroi, JP), Suzuki;
Yukitoshi (Hamamatsu, JP) |
Assignee: |
Yamaha Corporation
(Hamamatsu-shi, JP)
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Family
ID: |
40938175 |
Appl.
No.: |
12/317,692 |
Filed: |
December 23, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090200620 A1 |
Aug 13, 2009 |
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Foreign Application Priority Data
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Dec 28, 2007 [JP] |
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P2007-341426 |
Dec 28, 2007 [JP] |
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P2007-341440 |
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Current U.S.
Class: |
257/419;
257/E29.324 |
Current CPC
Class: |
B81B
3/0021 (20130101); H01L 29/84 (20130101); B81B
2203/0127 (20130101); B81B 2201/0257 (20130101); B81B
2203/04 (20130101) |
Current International
Class: |
H01L
29/84 (20060101) |
Field of
Search: |
;257/419,E29.324 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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9-508777 |
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Feb 2004 |
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JP |
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2004-506394 |
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Feb 2004 |
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JP |
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Other References
Tajima, et al., "Mechanical Properties of Capacitive Silicon
Microphone," MSS-01-34, Japanese Institute of Electrical Engineers
(Tohoku, Japan), p. 95-98. cited by other.
|
Primary Examiner: Dang; Trung
Attorney, Agent or Firm: Pillsbury Winthrop Shaw Pittman
LLP
Claims
What is claimed is:
1. An MEMS transducer comprising: a diaphragm having conductivity;
a plate having conductivity; a support structure for supporting the
diaphragm and the plate with a gap layer therebetween, wherein the
support structure has an interior wall surrounding the gap layer;
an electrode film having conductivity for covering a contact hole
formed in the support structure; and a protective film which is
formed on the support structure externally of the interior wall so
as to cover a side surface of the electrode film, wherein an
electric signal corresponding to variations of an electrostatic
capacitance formed between the diaphragm and the plate is output
via the electrode film.
2. An MEMS transducer according to claim 1, wherein the protective
film is composed of silicon nitride or silicon nitride oxide.
3. An MEMS transducer according to claim 2, wherein the support
structure has a multilayered structure including a silicon
substrate and a silicon oxide film which joins the silicon
substrate except for its periphery, and wherein the protective film
is formed in a region extended between the periphery of the silicon
substrate and a periphery of the silicon oxide film.
4. An MEMS transducer comprising: a diaphragm having conductivity;
a plate having conductivity; an insulating member for insulating
the diaphragm from the plate; an electrode film which is composed
of a conductive film so as to cover a contact hole formed in the
insulating member; and a protective film which is limitedly formed
in a part of a surface of the electrode film and a surrounding area
of the electrode film on a surface of the insulating member, thus
covering a side surface of the electrode film, wherein an electric
signal corresponding to variations of electrostatic capacitance
formed between the diaphragm and the plate is output from the
electrode film.
5. An MEMS transducer according to claim 4, wherein the protective
film is composed of silicon nitride or silicon nitride oxide.
6. An MEMS transducer according to claim 4, wherein the protective
film is formed on the surface of the electrode film except for its
center portion.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to MEMS (Micro Electro Mechanical
System) transducers applied to MEMS sensors used in miniature
condenser microphones, vibration sensors, pressure sensors, and
acceleration sensors, for example.
The present invention also relates to manufacturing methods of MEMS
transducers.
The present application claims priority on Japanese Patent
Application No. 2007-341440, and Japanese Patent Application No.
2007-341426, the contents of which are incorporated herein by
reference.
2. Description of the Related Art
Various types of miniature condenser microphones, which are
manufactured by use of manufacturing processes of semiconductor
devices, have been developed and disclosed in various documents
such as Patent Documents 1, 2, 3, and Non-Patent Document 1. Patent
Document 1: Japanese Patent Application Publication No. H09-50877
Patent Document 2: Japanese Patent Application Publication No.
2004-506394 Patent Document 3: U.S. Pat. No. 4,776,019 Non-Patent
Document 1: MSS-01-34 published by Japanese Institute of Electrical
Engineers
Condenser microphones have been referred to as MEMS microphones,
each of which is designed such that a diaphragm and a plate (which
are formed using thin films so as to form opposite electrodes of a
parallel-plate condenser) are separated from each other and are
supported above a substrate. When the diaphragm vibrates due to
sound waves, the displacement thereof occurs so as to vary
electrostatic capacitance of the condenser, so that variations of
electrostatic capacitance are converted into electric signals. MEMS
transducers serving as condenser microphones are covered with
protective films on the surfaces thereof, wherein through-holes are
formed in protective films so as to expose electrodes. The
protective films having insulating properties are used to protect
MEMS transducers from chemical corrosions (due to water, oxygen,
and sodium) and physical damages.
Relatively high stresses occur on deposited films composed of
nitride materials and nitrogen oxide materials which are deposited
on silicon substrates and silicon oxide films due to differences in
thermal expansion coefficients. When nitride materials and nitrogen
oxide materials are used for protective films, distortions may
occur in MEMS transducers having mechanical structures. This may
damage the mechanical functions of MEMS transducers.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an MEMS
transducer which is formed to protect electrodes thereof without
damaging mechanical functions thereof.
It is another object of the present invention to provide a
manufacturing method of the MEMS transducer.
In one embodiment of the present invention, an MEMS transducer is
constituted of a diaphragm having conductivity, a plate having
conductivity, and a support structure for supporting the diaphragm
and the plate with a gap layer therebetween wherein the support
structure has an interior wall surrounding the gap layer, an
electrode film having conductivity for covering a contact hole
formed in the support structure, and a protective film which is
formed on the support structure externally of the interior wall so
as to cover a side surface of the electrode film, wherein an
electric signal corresponding to variations of an electrostatic
capacitance formed between the diaphragm and the plate is output
via the electrode film.
Since the protective film (e.g. a pad protective film) is formed
externally of the interior wall of the support structure, it is
possible to prevent the diaphragm or the plate from being distorted
in shape due to the direct influence of the membrane stress of the
protective film; this makes it possible to form the protective film
by use of materials having high membrane stresses. The side surface
of the electrode film (e.g. a pad conductive film defining the
outline of a pad) is lowered in chemical stability because it is
activated in etching and because chemicals such as chloride and
fluorine may remain after etching. The present invention allows the
side surface of the electrode film having low chemical stability to
be covered with the protective film composed of high-protective
materials having high membrane stress. Thus, it is possible to
protect the electrode film without damaging the mechanical function
of the MEMS transducer.
It is preferable that the protective film be composed of silicon
nitride or silicon nitride oxide.
It is preferable that the support structure have a multilayered
structure including a silicon substrate and a silicon oxide film
(e.g. a surface insulating film), which joins the silicon substrate
except for its periphery, wherein the protective film is formed in
the region extended between the periphery of the silicon substrate
and the periphery of the silicon oxide film. This prevents movable
ions from entering into the edges of the joint surface between the
silicon substrate and the silicon oxide film.
In a manufacturing method adapted to the MEMS structure, the
diaphragm and the plate are supported with a gap layer therebetween
by means of the support structure having the interior wall
surrounding the gap layer; the contact hole is formed in the
support structure; the electrode film having conductivity is formed
to cover the contact hole; and then, the protective film is formed
to cover the side surface of the electrode film externally of the
interior wall of the support structure.
In another embodiment of the present invention, an MEMS transducer
is constituted of a diaphragm having conductivity, a plate having
conductivity, an insulating member for insulating the diaphragm
from the plate, an electrode film (e.g. a pad conductive film)
which is composed of a conductive film so as to cover a contact
hole formed in the insulating member, and a protective film (e.g. a
pad protective film) which is limitedly formed in a part of the
surface of the electrode film and the surrounding area of the
electrode film on the surface of the insulating member, thus
covering the side surface of the electrode film, wherein an
electric signal corresponding to variations of electrostatic
capacitance formed between the diaphragm and the plate is output
from the electrode film.
Since the protective film is formed in the limited surrounding area
of the electrode film, it is possible to use the material having
relatively high membrane stress for the protective film. The side
surface defining the outline of the electrode film may be lowered
in chemical stability due to dry etching and due to remaining
chemicals such as chloride and fluorine. In the present invention,
the side surface of the electrode film which bears low chemical
stability is covered with the protective film composed of nitrides
and nitric oxides having high protective properties; hence, it is
possible to reliably protect the electrode film without damaging
the mechanical function of the MEMS transducer.
Since the present invention prevents the distortion occurring due
to the protective film from being directly applied to the MEMS
transducer, it is possible to use silicon nitride and silicon
nitride oxide as the material of the protective film.
In the manufacturing method of the MEMS transducer, a contact hole
is formed in the insulating member; the electrode film is formed to
cover the contact hole of the insulating member; then, the
protective film is limitedly formed on a part of the surface of the
electrode film and the surrounding area of the electrode film on
the surface of the insulating member, thus covering the side
surface of the electrode film.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, aspects, and embodiments of the present
invention will be described in more detail with reference to the
following drawings.
FIG. 1 is a plan view showing a sensor die included in a condenser
microphone in accordance with a first embodiment of the present
invention.
FIG. 2 is a longitudinal sectional view taken along line A-A in
FIG. 1.
FIG. 3 is an exploded perspective view showing the film-laminating
structure of the sensor die.
FIG. 4A is a cross-sectional view showing the formation of a guard
terminal.
FIG. 4B is a cross-sectional view showing the formation of a plate
terminal.
FIG. 4C is a cross-sectional view showing the formation of a
substrate terminal.
FIG. 4D is a cross-sectional view showing the formation of a
diaphragm terminal.
FIG. 5 is a sectional view used for explaining a first step of a
manufacturing method of the condenser microphone.
FIG. 6 is a sectional view used for explaining a second step of the
manufacturing method of the condenser microphone.
FIG. 7 is a sectional view used for explaining a third step of the
manufacturing method of the condenser microphone.
FIG. 8 is a sectional view used for explaining a fourth step of the
manufacturing method of the condenser microphone.
FIG. 9 is a sectional view used for explaining a fifth step of the
manufacturing method of the condenser microphone.
FIG. 10 is a sectional view used for explaining a sixth step of the
manufacturing method of the condenser microphone.
FIG. 11 is a sectional view used for explaining a seventh step of
the manufacturing method of the condenser microphone.
FIG. 12 is a sectional view used for explaining an eighth step of
the manufacturing method of the condenser microphone.
FIG. 13A is a sectional view used for explaining a ninth step of
the manufacturing method of the condenser microphone.
FIG. 13B is a plan view of FIG. 13A, which is used for explaining
the relationship between pad protective films and pad conductive
films formed on a surface insulating film.
FIG. 14 is a sectional view used for explaining a tenth step of the
manufacturing method of the condenser microphone.
FIG. 15 is a sectional view used for explaining an eleventh step of
the manufacturing method of the condenser microphone.
FIG. 16 is a sectional view used for explaining a twelfth step of
the manufacturing method of the condenser microphone.
FIG. 17 is a sectional view used for explaining a thirteenth step
of the manufacturing method of the condenser microphone.
FIG. 18 is a sectional view used for explaining a fourteenth step
of the manufacturing method of the condenser microphone.
FIG. 19 is a sectional view used for explaining a fifteenth step of
the manufacturing method of the condenser microphone.
FIG. 20 is a sectional view used for explaining a sixteenth step of
the manufacturing method of the condenser microphone.
FIG. 21 is a sectional view used for explaining a seventeenth step
of the manufacturing method of the condenser microphone.
FIG. 22 is a sectional view of a sensor die according to a first
variation of the first embodiment.
FIG. 23 is a sectional view of a sensor die according to a second
variation of the first embodiment.
FIG. 24 is a sectional view of a sensor die according to a third
variation of the first embodiment.
FIG. 25 is a sectional view of a sensor die according to a fourth
variation of the first embodiment.
FIG. 26 is a plan view showing a sensor chip of a condenser
microphone in accordance with a second embodiment of the present
invention.
FIG. 27 is a sectional view showing the constitution of the sensor
chip.
FIG. 28 is an exploded perspective view showing the film-laminating
structure of the sensor chip.
FIG. 29A is a circuit diagram showing an equivalent circuit of the
sensor chip connected to the circuit chip.
FIG. 29B is a circuit diagram showing an equivalent circuit of the
sensor chip including a guard and connected to the circuit
chip.
FIG. 30 is a sectional view used for explaining a first step of a
manufacturing method of the sensor chip of the condenser
microphone.
FIG. 31 is a sectional view used for explaining a second step of
the manufacturing method of the sensor chip of the condenser
microphone.
FIG. 32 is a sectional view used for explaining a third step of the
manufacturing method of the sensor chip of the condenser
microphone.
FIG. 33 is a sectional view used for explaining a fourth step of
the manufacturing method of the sensor chip of the condenser
microphone.
FIG. 34 is a sectional view used for explaining a fifth step of the
manufacturing method of the sensor chip of the condenser
microphone.
FIG. 35 is a sectional view used for explaining a sixth step of the
manufacturing method of the sensor chip of the condenser
microphone.
FIG. 36 is a sectional view used for explaining a seventh step of
the manufacturing method of the sensor chip of the condenser
microphone.
FIG. 37 is a sectional view used for explaining an eighth step of
the manufacturing method of the sensor chip of the condenser
microphone.
FIG. 38 is a sectional view used for explaining a ninth step of the
manufacturing method of the sensor chip of the condenser
microphone.
FIG. 39 is a sectional view used for explaining a tenth step of the
manufacturing method of the sensor chip of the condenser
microphone.
FIG. 40 is a sectional view used for explaining an eleventh step of
the manufacturing method of the sensor chip of the condenser
microphone.
FIG. 41 is a sectional view used for explaining a twelfth step of
the manufacturing method of the sensor chip of the condenser
microphone.
FIG. 42 is a sectional view used for explaining a thirteenth step
of the manufacturing method of the sensor chip of the condenser
microphone.
FIG. 43 is a sectional view used for explaining a fourteenth step
of the manufacturing method of the sensor chip of the condenser
microphone.
FIG. 44 is a sectional view used for explaining a fifteenth step of
the manufacturing method of the sensor chip of the condenser
microphone.
FIG. 45 is a sectional view used for explaining a sixteenth step of
the manufacturing method of the sensor chip of the condenser
microphone.
FIG. 46 is a sectional view used for explaining a seventeenth step
of the manufacturing method of the sensor chip of the condenser
microphone.
FIG. 47 is a sectional view showing a prescribed part of the sensor
chip having undercuts due to anisotropic etching.
FIG. 48 is a sectional view showing another part of the sensor chip
having undercuts due to anisotropic etching.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be described in further detail by way of
examples with reference to the accompanying drawings.
1. First Embodiment
(1) Constitution
FIG. 1 shows a sensor die 1 of a condenser microphone (i.e. an
example of an MEMS transducer) in accordance with a first
embodiment of the preset invention. FIG. 2 is a sectional view take
along line A-A in FIG. 1 with respect to the sensor die 1; and FIG.
3 shows the film-laminating structure of the sensor die 1. For the
sake of simplification of illustrations, FIGS. 1 and 3 omit higher
layers (which are shown in FIG. 2) formed above an upper conductive
layer. The condenser microphone is constituted of the sensor die 1,
a circuit die (including a voltage supply and an amplifier, not
shown), and a package (not shown) for enclosing them.
The sensor die 1 is a movable component having a film-laminating
structure including a substrate 100, a lower insulating film 110, a
lower conductive film 120, an upper insulating film 130, an upper
conductive film 160, a surface insulating film 170, pad conductive
films 180, bump films 210, bump protective films 220, pad
protective films 190, and a plating protective film 200.
The substrate 100 is composed of P-type monocrystal silicon; but
this is not a restriction. The substrate 100 can be composed of any
types of materials having desired values of rigidity, thickness,
and strength for depositing thin films thereon and for supporting
structures constituted of thin films.
The lower insulating film 110 is formed on the substrate 100 and is
composed of silicon oxide (SiOx). The lower insulating film 110 is
used to form a ring-shaped portion (actually a rectangular portion
having a circular hole) 101, a plurality of diaphragm spacers 102
(which are aligned inside the ring-shaped portion 101), and a
plurality of guard insulators 103 (which are aligned inside the
ring-shaped portion 101).
The lower conductive film 120 is formed on the lower insulating
film 110 and is composed of polycrystal silicon entirely doped with
impurities such as phosphorus (P). The lower conductive film 120 is
used to form a guard 127 and a diaphragm 123.
The upper insulating film 130 is formed on the lower conductive
film 120 and the lower insulating film 110 and is composed of
silicon oxide. The upper insulating film 130 is used to form a
ring-shaped portion (actually a rectangular portion having a
circular hole) 132, and a plurality of plate spacers 131 (which are
aligned inside the ring-shaped portion 132).
The upper conductive film 160 is formed on the upper insulating
film 130 and is composed of polycrystal silicon entirely doped with
impurities such as phosphorus (P). The upper conductive film 160 is
used to form a plate 162 and an etching stopper ring 161.
The surface insulating film 170 is formed on the upper conductive
film 160 and the upper insulating film 130 and is composed of
silicon oxide.
The plating protective film 200 composed of silicon oxide is
exposed on the surface of the sensor die 1.
The sensor die 1 includes the diaphragm 123 and the plate 162 as
well as multilayered supports, and four terminals 125e, 162e, 123e,
and 100b.
Next, the constituent elements of the sensor die 1 will be
described below.
The diaphragm 123 is formed using the lower conductive film 120 and
is constituted of a center portion 123a, a plurality of arms 123c,
and a diaphragm lead 123d. The center portion 123a is supported in
parallel with the surface of the substrate 100 by means of the
diaphragm spacers 102 and is positioned to cover an opening 100a of
a back cavity C1 formed at the center of the substrate 100. The
arms 123c are extended externally from the center portion 123a in a
radial direction. Due to the formation of cutouts between the arms
123c, the rigidity of the diaphragm 123 is lower than the rigidity
of a foregoing diaphragm having no arm (not shown). In addition, a
plurality of diaphragm holes 123b is formed in the arms 123c, which
are thus lowered in rigidity. A gap layer C2 whose height
substantially matches the thickness of the diaphragm spacers 102 is
formed between the substrate 100 and the diaphragm 123. The gap
layer C2 is used to establish a balance between the internal
pressure of the back cavity C1 and the atmospheric pressure. The
diaphragm lead 123d is extended from the distal end of a prescribed
one of the arms 123c toward the diaphragm terminal 123e via a slit
of a guard ring 125c included in the guard 127 (see FIG. 3). Since
the diaphragm terminal 123e is short-circuited to the substrate
terminal 100b via the circuit die (not shown), the same potential
is set to the diaphragm terminal 123e and the substrate terminal
100b. A plurality of diaphragm bumps 123f is formed on the backside
of the diaphragm 123 positioned opposite to the surface of the
substrate 100. The diaphragm bumps 123f prevent the diaphragm 123
from being fixed to the substrate 100.
The plate 162 is supported in parallel with the diaphragm 123 by
the plate spacers 131 in such a way that the center thereof matches
the center of the diaphragm 123 in plan view. The plate 162 is
formed using the upper conductive film 160 and is constituted of a
center portion 162b, a plurality of arms 162a (which are extended
externally from the center portion 162b in a radial direction), and
a plate lead 162d. A plurality of plate holes 162c is formed in the
plate 162. The plate holes 162c allow an etchant (for use in
isotropic etching on the upper insulating film 130) to flow
therethrough. The remaining portion of the upper insulating film
130 after etching is used to form the plate spacers 131 and the
ring-shaped portion 132, while the other portion thereof (which is
removed by etching) is used to form a gap layer C3 between the
diaphragm 123 and the plate 162. The plate holes 162c are aligned
in consideration of the height of the gap layer C3, the shapes of
the plate spacers 131, and the etching speed. The plate lead 162d
whose width is smaller than the width of the arm 162a is extended
from the distal end of a prescribed one of the arms 162a of the
plate 162. The wiring path of the plate lead 162d overlaps the
wiring path of a guard lead 125d (see FIG. 3) in plan view. This
reduces the parasitic capacitance between the plate lead 162d and
the substrate 100. A plurality of projections (i.e. plate bumps)
162f is formed on the backside of the plate 162 positioned opposite
to the surface of the diaphragm 123. The plate bumps 162f are
formed using a silicon nitride (SiN) film (which joins the upper
conductive film 160 used for the formation of the plate 162) and a
polycrystal silicon film (joining the silicon nitride film). The
plate bumps 162f prevent the plate 162 from being fixed to the
diaphragm 123.
Next, the support structure for supporting the diaphragm 123 and
the plate 162 will be described in detail.
The support structure is constituted of the substrate 100, the
lower insulating film 110, the upper insulating film 130, the
surface insulating film 170, and the plating protective film
200.
A though-hole having the opening 100a is formed to run through the
substrate 100 in its thickness direction, thus forming the back
cavity C1 which is closed by a package substrate (not shown).
The diaphragm spacers 102 (which are formed using the lower
insulating film 110) are aligned with equal spacing therebetween in
the surrounding area of the opening 100a of the back cavity C1 in a
circumferential direction. The diaphragm spacers 102 support the
diaphragm 123 above the substrate 100 via a gap layer C2 while
insulating the diaphragm 123 from the plate 162.
The plate spacers 131 (which are formed using the upper insulating
film 130) join guard electrodes 125a (which are formed using the
lower conductive layer 120). The plate spacers 131 support the
plate 162 above the diaphragm 123 with the gap layer C3
therebetween. The plate spacers 131 are positioned in the cutouts
formed between the adjacent arms 123a of the diaphragm 123. The
guard electrodes 125a are supported above the substrate 100 via the
guard insulators 103 (which are formed using the lower insulating
film 110). That is, the plate 162 is supported above the substrate
100 by means of the guard insulators 103, the guard electrodes
125a, and the plate spacers 131.
The gap layer C3 formed between the diaphragm 123 and the plate 162
is surrounded by an interior wall 132a of the ring-shaped portion
132 of the upper insulating film 130.
Next, the terminal structure of the sensor die 1 of the condenser
microphone will be described with reference to FIGS. 4A to 4D.
The sensor die 1 is equipped with the four terminals 125e, 162e,
123e, and 100b, all of which are formed using the pad conductive
films 180, the bump films 210, and the bump protective films 220 as
shown in FIGS. 4A to 4D.
The pad conductive films 180 are mainly composed of aluminum. The
pad conductive films 180 contain silicon at 1% in order to prevent
silicon materials from being diffused from the upper conductive
film 160 to the pad conductive films 180. As shown in FIG. 4A, the
pad conductive film 180 covering a contact hole CH3 join the guard
lead 125d. As shown in FIG. 4B, the pad conductive film 180
covering a contact hole CH2 joins the plate lead 162d. As shown in
FIG. 4C, the pad conductive film 180 covering a contact hole CH4
joins the substrate 100. As shown in FIG. 4D, the pad conductive
film 180 covering a contact hole CH1 joins the diaphragm lead
123d.
The pad protective films 190 are formed on the surface insulating
film 170 and the pad conductive films 180 so as to cover the side
surfaces of the pad conductive films 180 (which are terminal
surfaces formed by way of etching). The pad protective films 190
are composed of silicon nitride or silicon oxide nitride. As shown
in FIG. 13B, the pad protective films 190 are formed to surround
the pad conductive films 180 on the surface of the surface
insulating film 170 forming the support structure. Specifically,
the pad protective films 190 are each formed in the limited areas
externally of the interior wall 132a of the ring-shaped portion
132, i.e. the area of the pad conductive film 180 except for a part
of its surface and the area surrounding the pad conductive film 180
on the surface of an insulating member 171 (corresponding to the
upper insulating film 130), thus covering the side surfaces of the
pad conductive films 180. The pad protective films 190 are isolated
from each other and are formed in connection with the terminals
125e, 162e, 123e, and 100b. That is, the pad conductive films 190
cover only the limited areas of the sensor die 1, wherein they do
not cover the plate 162. For this reason, even though the pad
protective films 190 are composed of silicon nitride having a
relatively high membrane stress, they do not damage the function of
the sensor die 1. If the pad protective films 190 are formed
inwardly of the interior wall 132a of the ring-shaped portion 132,
the pad protective films 190 are inevitably connected to the plate
162, which is thus distorted in shape. This varies the height of
the gap layer C2 between the plate 162 and the diaphragm 123, thus
damaging the function of the sensor die 1 and causing dispersions
in characteristics of the sensor die 1.
The bump films 210 are formed in the prescribed areas of the
surfaces of the pad conductive films 180 which are not covered with
the pad protective films 190. In other words, the pad protective
films 190 are formed on the prescribed areas of the surfaces of the
pad conductive films 180 except for their bump forming areas. The
bump films 210 are composed of nickel.
The surfaces of the bump films 210 are covered with the bump
protective films 220, which are exposed on the surface of the
sensor die 1. The bump films 220 are composed of metals having
superior corrosion resistances such as gold (Au).
The guard 127 is constituted of the guard electrodes 125a, the
guard connectors 125b, the guard ring 125c, and the guard lead
125d. The guard 127 reduces the parasitic capacitance formed
between the diaphragm 123 and the plate 162.
(2) Operation
A bias voltage which is stabilized by a charge pump installed in
the circuit die is applied to the diaphragm 123. Sound waves
entering into the through-hole of a package (not shown) are
transmitted to the diaphragm 123 via the plate holes 162c and the
cutouts between the arms 162a of the plate 162. Since sound waves
having the same phase are propagated along both the surface and the
backside of the plate 162, the plate 162 does not vibrate
substantially. Sound waves reaching the diaphragm 123 cause
vibration of the diaphragm 123. Vibrating the diaphragm 123 varies
the electrostatic capacitance of a parallel-plate condenser whose
opposite electrodes correspond to the pate 162 and the diaphragm
123. Electric signals corresponding to variations of the
electrostatic capacitance formed between the plate 162 and the
diaphragm 123 are picked up as potential differences occurring
between the diaphragm terminal 123e and the plate terminal 162e,
whereby they are output from the sensor die 1. Electric signals
representative of voltages are amplified by an amplifier (not
shown) of the circuit die. That is, electric signals corresponding
to variations of the electrostatic capacitance between the plate
162 and the diaphragm 123 are output via the pad conductive films
180 forming the diaphragm terminal 123e and the plate terminal
162e. In this connection, the charge pump and the amplifier can be
installed in the sensor die 1.
(3) Manufacturing Method
Next, the manufacturing method of the condenser microphone will be
described with reference to FIGS. 5 to 12, 13A, 13B, and 14 to
21.
In a first step of the manufacturing method shown in FIG. 5, the
lower insulating film 110 which is a deposited film composed of
silicon oxide is formed on the entire surface of the substrate 100.
Dimples 110a used for the formation of the diaphragm bumps 123f are
formed in the lower insulating film 110 by way of etching using a
photoresist mask. The lower conductive film 120 which is a
deposited film composed of polycrystal silicon is formed on the
surface of the lower insulating film 110 by way of Chemical Vapor
Deposition (CVD). Thus, the diaphragm bumps 123e are formed below
the dimples 110a. In addition, etching using a photoresist mask is
performed on the lower conductive film 120 so as to form the
diaphragm 123 and the guard 127 (both of which are formed using the
lower conductive film 120).
In a second step of the manufacturing method shown in FIG. 6, the
upper insulating film 130 which is a deposited film composed of
silicon oxide is formed on the entire surfaces of the lower
insulating film 110 and the lower conductive film 120. Dimples 130a
used for the formation of the plate bumps 162f are formed in the
upper insulating film 130 by way of etching using a photoresist
mask.
In a third step of the manufacturing method shown in FIG. 7, the
plate bumps 162f each of which is composed of the polycrystal
silicon film 135 and the silicon nitride film 135 are formed on the
surface of the upper insulating film 130. The silicon nitride film
136 prevents the diaphragm 123 from being brought into contact with
and short-circuited to the plate 162.
In a fourth step of the manufacturing method shown in FIG. 8, the
upper conductive film 160 which is a deposited film composed of
polycrystal silicon is formed on the surface of the upper
insulating film 130 and the surface of the silicon nitride film 136
by way of CVD. The upper conductive film 160 is etched using a
photoresist mask so as to form the plate 162 and the etching
stopper ring 161. In this step, the plate holes 162c are not formed
in the plate 162.
In a fifth step of the manufacturing method shown in FIG. 9,
through-holes H1, H3, and H4 (which correspond to the contact holes
CH1, CH3, and CH4) are formed in the lower insulating film 110 and
the upper insulating film 130 by way of anisotropic etching using a
photoresist mask, thus partially exposing the diaphragm 123, the
guard 127, and the substrate 100.
In a sixth step of the manufacturing method shown in FIG. 10, the
surface insulating film 170 composed of silicon oxide is formed on
the entire surface by way of plasma CVD. Etching is performed using
a photoresist mask so as to completely form the contact holes CH1,
CH2, CH3, and CH4 in the surface insulating film 170. As a result,
the diaphragm 123, the plate 162, the guard 127, and the substrate
100 are partially exposed.
In a seventh step of the manufacturing method shown in FIG. 11, a
deposited film composed of AlSi is formed on the entire surface by
way of sputtering so as to cover the contact holes CH1, CH2, CH3,
and CH4, wherein it joins the diaphragm 123, the plate 162, the
guard 127, and the substrate 100. Then, etching using a photoresist
mask is performed to partially remove the deposited film of AlSi
while leaving the prescribed portions of the deposited film
covering the contact holes CH1, CH2, CH3, and CH4, by which the pad
conductive films 180 are formed using the deposited film of AlSi.
At this time, the deposited film of AlSi is subdivided into plural
areas corresponding to the contact holes CH1, CH2, CH3, and CH4,
thus defining the outlines of the pad conductive films 180, i.e.
the side surfaces of the pad conductive films 180. The deposited
film of AlSi is subjected to patterning by wet etching by use of a
spin processor under prescribed conditions, for example, in which a
mixed acid (e.g. a mixed acid of phosphoric acid, nitric acid, and
water) is used as an etchant, a heating temperature is set to a
range from 60.degree. C. to 75.degree. C. (preferably, it is set to
65.degree. C.), and spinning is performed for a processing time
ranging from 30 sec to 120 sec (preferably, a processing time of 60
sec) with a rotation speed ranging from 600 rpm to 1,000 rpm
(preferable, a rotation speed of 800 rpm). In wet etching, the
activated side surfaces may be exposed on the pad conductive films
180, which thus suffer from lower chemical stability and corrosion
easily. In dry etching, the side surfaces of the pad conductive
films 180 are exposed to chlorine gas used in etching so that the
pad conductive films 180 suffer from low chemical stability and
corrosion easily.
To cope with the above drawback, in an eighth step of the
manufacturing method shown in FIG. 12, the pad protective films 190
which are deposited films composed of silicon nitride and which are
used to protect the side surfaces of the pad conductive films 180
are formed on the surface of the surface insulating film 170 and
the surfaces of the pad conductive films 180 by way of low-stress
plasma CVD under prescribed conditions at temperature of
400.degree. C., pressure of 2.5 Torr, SiH.sub.4 flow of 0.3 SLM,
NH.sub.3 flow of 1.75 SLM, bias power (RF H/F) of 0.44 kW/0.351 kW.
Thus, the pad protective films 190 composed of silicon nitride are
formed with the thickness of 1.6 .mu.m by way of low-stress plasma
CVD.
In a ninth step of the manufacturing method shown in FIGS. 13A and
13B, dry etching is performed using a photoresist mask to partially
remove the pad protective films 190 while leaving the peripheral
portions surrounding the pad conductive films 180. As a result, the
pad protective films 190, which are deposited films composed of
silicon nitride having superior protection characteristics, are
isolated from each other and are formed in connection with the
contact holes CH1, CH2, CH3, and CH4. Specifically, the patterning
of the pad protective films 190 is achieved by way of dry etching
using a parallel-plate plasma etcher under prescribed conditions at
CF4+O2 mixed gas flow of 150 SCCM, pressure ranging from 0.8 Torr
to 1.2 Torr (preferably, pressure of 1.0 Torr), bias power of 250
W, and heating temperature of 80.degree. C. for 130 seconds. In the
present embodiment, the patterning is performed so as to localize
the pad protective films 190 composed of silicon oxide; hence, it
is possible to suppress the distortion of the sensor die 1 due to
the stress of the pad protective films 190.
In an tenth step of the manufacturing method shown in FIG. 14,
anisotropic etching is performed using a photoresist mask so as to
form through-holes corresponding to the plate holes 162c in the
surface insulating film 170, whereby the plate holes 162c are
formed in the upper conductive film 160. This step is consecutively
executed by using the surface insulating film 170 having
through-holes as a mask for use in etching of the upper conductive
film 160.
In an eleventh step of the manufacturing method shown in FIG. 15,
the plating protective film 200 composed of silicon oxide is formed
on the surface of the surface insulating film 170, the surfaces of
the pad conductive films 180, and the surfaces of the pad
protective films 190. The plating protective film 200 is patterned
by etching using a photoresist mask so as to expose the center
areas of the surfaces of the pad conductive films 180 covering the
contact holes CH1, CH2, CH3, and CH4.
In a twelfth step of the manufacturing method shown in FIG. 16, the
bump films 210 composed of nickel are formed on the exposed
surfaces of the pad conductive films 180, which are exposed in the
through-holes of the plating protective film 200 by way of
electroless plating. The bump protective films 220 composed of Au
are formed on the surfaces of the bump films 210. Then, the
backside of the substrate 100 is polished so as to achieve the
prescribed thickness of an actual product.
In a thirteenth step of the manufacturing method shown in FIG. 17,
etching is performed using a photoresist mask so as to form a
ring-shaped channel H5, which exposes the etching stopper ring 161
in connection with the plating protective film 200 and the surface
insulating film 170.
In a fourteenth step of the manufacturing method shown in FIG. 18,
a photoresist mask R1 having a through-hole H6 (used for the
formation of the back cavity C1 of the substrate 100) is formed on
the backside of the substrate 100.
In a fifteenth step of the manufacturing method shown in FIG. 19,
the through-hole corresponding to the back cavity C1 is formed in
the substrate 100 by way of Deep Reactive Ion Etching (Deep-RIE),
wherein the lower insulating film 110 serves as an etching
stopper.
In sixteenth and seventeenth steps of the manufacturing method
shown in FIGS. 20 and 21, isotropic etching is performed using a
photoresist mask R2 and buffered hydrofluoric acid (BHF) so as to
remove the plating protective film 200 and the surface insulating
film 170 exposed in the through-hole H6 of the photoresist mask R2
while further removing a part of the upper insulating film 130 so
as to form the ring-shaped portion 132, the plate spacers 131, and
the gap layer C3. In addition, a part of the lower insulating film
110 is removed from the back cavity C1 so as to form the guard
insulators 103, the diaphragm spacers 102, the ring-shaped portion
101, and the gap layer C2. At this time, an etchant (i.e. BHF)
enters into the through-hole H6 of the photoresist mask R2 and the
opening 100a of the substrate 100. The outline of the upper
insulating film 130 is defined by the plate 162. That is, the
ring-shaped portion 132 and the plate spacers 131 are formed by way
of self-alignment of the plate 162. The outline of the lower
insulating film 110 is defined by the opening 100a of the substrate
100, the diaphragm 123, the guard electrodes 125a, the guard
connectors 125b, and the guard ring 125c, wherein the guard
insulators 103 and the diaphragm spacers 102 are formed by way of
self-alignment.
Lastly, the photoresist mask R2 is removed; then, the substrate 100
is subjected to dicing, thus completing production of the sensor
die 1 shown in FIG. 2 for use in a condenser microphone. The sensor
die 1 is attached to a package substrate (not shown) together with
the circuit die; the terminals 125e, 162e, 123e, and 100b of the
sensor die 1 are electrically connected to respective terminals
(not shown) of the circuit die; then, a package cover (not shown)
is placed on the package substrate, thus completely forming the
condenser microphone. When the sensor die 1 is attached to the
package substrate, the opening of the back cavity C1 in the
backside of the substrate 100 is closed by the package
substrate.
(4) Variations
The present embodiment can be modified in a variety of ways; hence,
variations will be described with reference to FIGS. 22 to 25.
FIGS. 22, 23, 24, and 25 show sensor dies 2, 3, 4, and 5 each for
use in a condenser microphone, wherein parts identical to those of
the sensor die 1 shown in FIG. 2 are designated by the same
reference numerals. As shown in FIG. 22, it is possible to cover
the peripheral portion of the substrate 100 and the peripheral
portion of the surface insulating film 170 with the pad protective
films 190, whereby the edges of the joint surface between the
substrate 100 (composed of monocrystal silicon) and the surface
insulating film 170 (composed of silicon oxide) are covered with
the pad protective films 190 which is composed of silicon nitride
or silicon nitride oxide, the protective function of which is
higher than that of the plating protective film 200. This reliably
prevents movable ions from entering into the edges of the joint
surface between the substrate 100 composed of monocrystal silicon
and the surface insulating film 170 composed of silicon oxide.
In the above, it is preferable that the pad protective films 190 be
formed in narrow regions as possible as long as they cover the side
surfaces of the pad conductive films 180 serving as electrode
films. In this sense, it is possible to integrally unify the pad
protective films 190 with respect to the combination of the
adjacent terminals 123e and 100b and the combination of the
adjacent terminals 125e and 162e as shown in FIGS. 22 to 25.
Alternatively, it is possible to unify the pad protective films 190
so as to integrally cover all the terminals 125e, 162e, 123e, and
100b. Alternatively, it is possible to extend the pad protective
film 190 above or in proximity to the etching stopper ring 161 as
shown in FIG. 23, wherein the pad protective film 190 is formed on
the entire surface externally of the interior wall 132a of the
ring-shaped portion 132 forming the support structure. In this
connection, the pad protective films 190 can be each formed in a
circular shape or a polygonal ring shape.
2. Second Embodiment
A sensor chip 10 of a condenser microphone according to a second
embodiment of the present invention will be described with
reference to FIGS. 26 to 28, FIGS. 29A and 29B, and FIGS. 30 to 48,
wherein parts identical to those of the sensor die 1 of the
condenser microphone shown in FIGS. 1 to 3, FIGS. 4A to 4D, FIGS. 5
to 12, FIGS. 13A and 13B, and FIGS. 14 to 25 are designated by the
same reference numerals; hence, duplicate descriptions thereof are
simplified as necessary.
(1) Constitution
FIG. 26 shows the constitution of the sensor chip 10, which is an
MEMS structure of the condenser microphone according to the second
embodiment of the present invention; FIG. 27 is a sectional view of
the sensor chip 10; and FIG. 28 is an exploded perspective view
showing the film-laminating structure in the sensor chip 10. The
condenser microphone (serving as an MEMS transducer) is constituted
of the sensor chip 10, a circuit chip (including a power circuit
and an amplifier, not shown), and a package (not shown).
First, the film-laminating structure of an MEMS structure of the
sensor chip 10 will be described below.
The sensor chip 10 is constituted of the lower insulating film 110,
the lower conductive film 120, the upper insulating film 130, the
upper conductive film 160, and the surface insulating film 170, all
of which are laminated and deposited on the substrate 100.
The opening 100a of the through-hole of the substrate 100 composed
of P-type monocrystal silicon forms the opening of the cavity
C1.
The insulating member 171 is constituted of the surface insulating
film 170 and the upper insulating film 130 (which insulates the
upper conductive film 160 from the lower conductive film 120).
The lower insulating film 110, which joins the substrate 100, the
lower conductive film 120, and the upper insulating film 130, is
composed of silicon oxide (SiOx). The lower insulating film 110 is
used to form the diaphragm spacers 102 which are circumferentially
aligned with equal spacing therebetween, the guard spacers (or
guard insulators) 103 which are circumferentially aligned with
equal spacing therebetween inwardly of the diaphragm spacers 102,
and the ring-shaped portion 101 which insulates the guard ring 125c
and the guard lead 125d from the substrate 100.
The lower conductive film 120 joining the lower insulating film 110
and the upper insulating film 130 is composed of polycrystal
silicon entirely doped with impurities such as phosphorus (P). The
lower conductive film 120 is used to form the diaphragm 123 and the
guard 127 including the guard electrodes 125a, the guard connectors
125b, the guard ring 125c, and the guard lead 125d.
The upper insulating film 130 joining the lower conductive film
120, the upper conductive film 160, and the lower insulating film
110 is composed of silicon oxide so as to form a part of the
insulating member 171. The upper insulating film 130 is used to
form the plate spacers 131, which are circumferentially aligned
with equal spacing therebetween, and the ring-shaped portion 132,
which is positioned externally of the plate spacers 131 and which
supports the etching stopper ring 161 while insulating the plate
lead 162d from the guard lead 125d.
The upper conductive film 160 joining the upper insulating film 130
is composed of polycrystal silicon entirely doped with impurities
such as phosphorus (P). The upper conductive film 160 is used to
form the plate 162, the plate lead 162d, and the etching stopper
ring 161.
The surface insulating film 170 joining the upper conductive film
160 and the upper insulating film 130 is composed of silicon oxide
so as to form a part of the insulating member 171.
The MEMS structure of the sensor chip 10 has the four terminals
125e, 162e, 123e, and 100b, all of which are formed using the pad
conductive films 180 (composed of metals), the bump films 210, and
the bump protective films 220. The pad conductive films 180 are
composed of aluminum, wherein it may contain silicon at 1% in order
to prevent silicon from being diffused from the upper conductive
film 160 to the pad conductive films 180. The pad conductive films
180 cover the contact holes CH1, CH2, CH3, and CH4 (which are
formed in the upper conductive film 160 and the surface insulating
film 170), wherein the peripheries and side surfaces thereof are
covered with the pad protective films 190 composed of silicon
nitride. The pad protective films 190 are formed only in the
surrounding areas of the pad conductive films 180 on the surface of
the surface insulating film 170 (which forms the surface of the
insulating member 171). That is, the pad protective films 190 are
formed in the limited areas, i.e. the surfaces of the pad
conductive films 180 except for center portions and the surrounding
areas of the pad conductive films 180 on the surface of the
insulating member 171, thus covering the "activated" side surfaces
of the pad conductive films 180. The pad protective films 190 are
isolated from each other in connection with the terminals 125e,
162e, 123e, and 100b, wherein they cover the limited area of the
MEMS structure of the sensor chip 10 but does not cover the movable
portions of the pad protective films 190. For this reason, even
though the pad protective films 190 are composed of silicon nitride
causing a relatively high membrane stress, it is possible to
prevent the pad protective films 190 from damaging the mechanical
function of the MEMS structure of the sensor chip 10. The bump
films 210 having conductivity composed of Ni are formed on the
center portions of the surfaces of the pad conductive films 180
which are not covered with the pad protective films 190. In short,
the pad protective films 190 are formed on the surfaces of the pad
conductive films 180 except for bump forming regions. The surfaces
of the bump films 210 are covered with the bump protective films
220 composed of Au having conductivity and relatively high
corrosion resistance. The side surfaces of the pad conductive films
180 which are activated due to patterning are adequately protected
by the pad protective films 190 composed of silicon nitride. It is
possible to bond wires to the pad conductive films 180. In this
connection, the pad protective films 190 can be composed of silicon
nitride oxide, for example.
Next, the mechanical structure of the MEMS structure of the sensor
chip 10 will be described below.
The diaphragm 123 is a single layer having conductivity in the
entirety, i.e. a thin silicon film, wherein it is constituted of
the center portion 123a and the arms 123c. The diaphragm 123 is
supported between the substrate 100 and the plate 162 with
prescribed gaps therebetween by means of the diaphragm spacers 102
having pillar shapes which join the distal ends of the arms 123c,
wherein the diaphragm 123 is positioned in parallel with the
surface of the substrate 100 while being insulated from the plate
162. The diaphragm 123 is reduced in rigidity due to the cutouts
formed between the adjacent arms 123c in comparison with the
foregoing diaphragm having no arm and no cutout. A plurality of
diaphragm holes 123b is formed in each of the arms 123c, which are
thus reduced in rigidity.
The diaphragm spacers 102 are circumferentially aligned with equal
spacing therebetween in the surrounding area of the opening 100a of
the back cavity C1. The diaphragm spacers 102 are insulating
deposited films having pillar shapes. The diaphragm 123 is
supported above the substrate 100 via the diaphragm supports 102
such that the center portion 123a covers the opening 100a of the
back cavity C1. The gap layer C2 whose height substantially matches
the thickness of the diaphragm spacers 102 is formed between the
substrate 100 and the diaphragm 123, thus establishing a balance
between the internal pressure of the back cavity C1 and the
atmospheric pressure. The gap layer C2 has a small width and a long
length elongated in the radial direction of the diaphragm 123 so as
to form the maximum acoustic resistance in the path for propagating
sound waves (for vibrating the diaphragm 123) toward the opening
100a of the back cavity C1.
A plurality of diaphragm bumps 123f is formed on the backside of
the diaphragm 123 positioned opposite to the surface of the
substrate 100. The diaphragm bumps 123f are projections for
preventing the diaphragm 123 from being fixed to the substrate 100,
wherein they are formed using the waviness of the lower conductive
film 120 forming the diaphragm 123.
The diaphragm 123 is connected to the diaphragm terminal 123e via
the diaphragm lead 123d which is elongated from the distal end of
the prescribed arm 123c so as to join the pad conductive film 180
applied to the diaphragm terminal 123e. The width of the diaphragm
lead 123d is smaller than the width of the arm 123c and is formed
using the lower conductive film 120 which is also used to form the
diaphragm 123. The diaphragm lead 123d is extended toward the
diaphragm terminal 123e via the slit of the guard ring 125c. Since
the diaphragm terminal 123e and the substrate terminal 100b are
short-circuited to the circuit chip (not shown) as shown in FIGS.
29A and 29B, both the diaphragm 123 and the substrate 100 are set
to the same potential.
When the potential of the diaphragm 123 differs from the potential
of the substrate 100, a parasitic capacitance occurs between the
diaphragm 123 and the substrate 100. However, since the diaphragm
123 is supported by the diaphragm spacers 102 having air layers
therebetween, it is possible to reduce the parasitic capacitance in
comparison with the foregoing structure in which the diaphragm is
supported by the spacer having a ring-shaped wall structure.
The plate 162 is a single thin film having conductivity in the
entirety, wherein it is constituted of the center portion 162b and
the arms 162a. The plate 162 is supported by the plate spacers 162
having pillar shapes which join the distal ends of the arms 162a.
The plate 162 is positioned in parallel with the diaphragm 123 such
that the center of the plate 162 overlaps the center of the
diaphragm 123 in plan view. The shortest distance between the
center to the periphery of the plate 162 is shorter than the
shortest distance between the center to the periphery of the
diaphragm 123; hence, the plate 162 does not face the periphery of
the diaphragm 123 (whose amplitude of vibration is very small). The
cutouts formed between the adjacent arms 162a of the plate 162 are
positioned in proximity to but do not face the periphery of the
diaphragm 123 in plan view, wherein the arms 123c are extended in
the cutouts of the arms 162a in plan view. This increases the
length of the diaphragm 123 (i.e. the distance between both ends of
the diaphragm 123 causing vibration) without increasing the
parasitic capacitance between the diaphragm 123 and the plate
162.
A plurality of plate holes 162c is formed in the plate 162, wherein
it collectively functions as a passage for propagating sound waves
to the diaphragm 123, and it also collectively functions as a
through-hole for transmitting an etchant (used for isotropic
etching of the upper insulating film 130) therethrough. The
remaining portion of the upper insulating film 130 after etching is
used to form the plate spacers 131 and the ring-shaped portion 132,
while the removed portion thereof forms the gap layer C3 between
the diaphragm 123 and the plate 162. The plate holes 162c are
aligned in consideration of the height of the gap layer C3, the
shapes of the plate holes 131, and the etching speed. As the
distance between the adjacent plate holes 162c becomes smaller, the
width of the ring-shaped portion 132 of the upper insulating film
130 becomes correspondingly smaller, thus reducing the overall chip
area. However, the rigidity of the plate 162 becomes lower as the
distance between the adjacent plate holes 162c becomes smaller.
The plate spacers 131 join the guard electrodes 125a which are
positioned in the same layer as the diaphragm 123, wherein the
guard electrodes 125a are formed using the lower conductive film
120 which is also used to form the diaphragm 123. The plate spacers
131 are formed using the upper insulating film 130 which is an
insulating deposited film joining the plate 162. The plate spacers
131 are circumferentially aligned in the surrounding area of the
opening 100a of the back cavity C1. Since the plate spacers 131 are
positioned in the cutouts between the arms 123c of the diaphragm
123, it is possible to reduce the maximum diameter of the plate 162
to be smaller than the maximum diameter of the diaphragm 123. This
reduces the parasitic capacitance between the plate 162 and the
substrate 100 while increasing the rigidity of the plate 162.
The plate 162 is supported above the substrate 100 via a plurality
of spacers 129 having pillar shapes which are constituted of the
guard spacers 103, the guard electrodes 125a, and the plate spacers
131. The spacers 129 form the gap layer C3 between the plate 162
and the diaphragm 123, so that the gap layers C2 and C3 are formed
between the plate 162 and the substrate 100. Since both the guard
spacers 103 and the plate spacers 131 have insulating properties,
the plate 162 is insulated from the substrate 100.
When the potential of the plate 162 differs from the potential of
the substrate 100 without the intervention of the guard electrodes
125a, a parasitic capacitance occurs between the plate 162 and the
substrate 100. This parasitic capacitance is increased by an
insulator inserted between the plate 162 and the substrate 100 (see
FIG. 29A). The second embodiment is designed such that the plate
162 is supported above the substrate 100 via the spacers 129 having
pillar shapes which are constituted of the guard spacers 103, the
guard electrodes 125a, and the plate spacers 131 and which are
distanced from each other; hence, it is possible to reduce the
parasitic capacitance in the MEMS structure having no guard
electrode 125a in comparison with the foregoing structure in which
the plate is supported above the substrate via an insulator having
a ring-shaped wall structure.
A plurality of plate bumps 162f is formed on the backside of the
plate 162 positioned opposite to the surface of the diaphragm 123.
Each of the plate bumps 162f is formed using a silicon nitride
(SiN) film (which joins the upper conductive film 160 forming the
plate 162) and a polycrystal silicon film (which joins the silicon
nitride film). The plate bumps 162f prevent the plate 162 from
being fixed to the diaphragm 123.
The plate lead 162d whose width is smaller than the width of the
arm 162a is extended from the distal end of the prescribed arm 162a
of the plate 162 to the pate terminal 162e, so that it joins the
pad conductive film 180 applied to the plate terminal 162e. The
plate lead 162d is formed using the upper conductive film 160
(which is also used to form the plate 162), wherein the wiring path
of the plate lead 162d overlaps the wiring path of the guard lead
125d in plan view. This reduces the parasitic capacitance between
the plate lead 162d and the substrate 100.
(2) Operation
Next, the operation of the sensor chip 10 will be described with
reference to FIGS. 29A and 29B, each of which shows the circuitry
for connecting the sensor chip 10 to the circuit chip. A charge
pump CP of the circuit chip applies a stabilized bias voltage to
the diaphragm 123. As the bias voltage becomes higher, the
sensitivity of the condenser microphone becomes correspondingly
higher while the diaphragm 123 may be easily fixed to the plate
162, wherein the rigidity of the plate 162 is an important factor
in the design of the condenser microphone.
Sound waves (which enter into the through-hole of a package, not
shown) are propagated through the plate holes 162c and the cutouts
between the arms 162a of the plate 162 so as to reach the diaphragm
123. Since sound waves having the same phase are propagated along
the surface and the backside of the plate 162, the plate 162 does
not vibrate substantially. Sound waves reaching the diaphragm 123
cause vibration relative to the plate 162. When the diaphragm 123
vibrates due to sound waves, the electrostatic capacitance of a
parallel-plate condenser whose opposite electrodes correspond to
the plate 162 and the diaphragm 123 is varied; then, electric
signals corresponding to variations of electrostatic capacitance
are output from the sensor chip 10 as voltage differences occurring
between the diaphragm terminal 123e and the plate terminal 162e. An
amplifier A of the circuit chip amplifies electric signals
representing voltages. That is, electric signals corresponding to
variations of electrostatic capacitance between the plate 162 and
the diaphragm 123 are output via the pad conductive films 180
applied to the diaphragm terminal 123e and the plate terminal 162e.
Since an output signal of the sensor chip 10 has a high impedance,
it is necessary to incorporate the amplifier A inside the
package.
The circuit elements such as the charge pump P and the amplifier A
can be incorporated into the sensor chip 10, thus making the
condenser microphone have a single-chip structure.
(3) Manufacturing Method
Next, a manufacturing method of the sensor chip 10 of the condenser
microphone will be described with reference to FIGS. 30 to 46.
In a first step of the manufacturing method shown in FIG. 30, the
lower conductive film 110 composed of silicon oxide is formed on
the entire surface of the substrate 100. The dimples 110a used for
the formation of the diaphragm bumps 123f are formed in the lower
conductive film 110 by way of etching using a photoresist mask. The
lower conductive film 120 composed of polycrystal silicon is formed
on the surface of the lower insulating film 110 by way of Chemical
Vapor Deposition (CVD), whereby the diaphragm bumps 123f are formed
below the dimples 110a. Thereafter, the lower conductive film 120
is etched using a photoresist mask so as to form the diaphragm 123
and the guard 127.
In a second step of the manufacturing method shown in FIG. 31, the
upper insulating film 130 composed of silicon oxide is formed on
the entire surface of the lower insulating film 110 and the lower
conductive film 120. The dimples 130a used for the formation of the
plate bumps 162f are formed in the upper insulating film 130 by way
of etching using a photoresist mask.
In a third step of the manufacturing method shown in FIG. 32, the
plate bumps 162f are formed on the surface of the upper insulating
film 130 by use of the polycrystal silicon film 135 and the silicon
nitride film 136. Herein, the silicon nitride film 136 is formed
after the patterning of the polycrystal silicon film 135 in
accordance with the known method; hence, the exposed surface of the
polycrystal silicon film 135 (used for the formation of the dimples
130a) is entirely covered with the silicon nitride film 136. The
silicon nitride film 136 is an insulating film which prevents the
plate 162 from being short-circuited to the diaphragm 123 even when
the diaphragm 123 is fixed to the plate 162.
In a fourth step of the manufacturing method shown in FIG. 33, the
upper conductive film 160 composed of polycrystal silicon is formed
on the exposed surface of the upper insulating film 130 and the
surface of the silicon nitride film 136 by way of CVD. The upper
conductive film 160 is etched using a photoresist mask so as to
form the plate 162, the plate lead 162d, and the etching stopper
ring 161. In this step, the plate holes 162c are not formed in the
plate 162.
In a fifth step of the manufacturing method shown in FIG. 34, the
through-holes H1, H3, and H4 (used for the formation of the contact
holes CH1, CH3, and CH4) are formed in the lower insulating film
110 and the upper insulating film 130 by way of anisotropic
etching.
In a sixth step of the manufacturing method shown in FIG. 35, the
surface insulating film 170 composed of silicon oxide is formed on
the entire surface by way of plasma CVD. Then, etching is performed
using a photoresist mask so as to form the contact holes CH1, CH2,
CH3, and CH4 in the surface insulating film 170.
In a seventh step of the manufacturing method shown in FIG. 36, an
AlSi film is formed on the entire surface so as to cover the
contact holes CH1, CH2, CH3, and CH4 by way of sputtering. Then,
etching is performed using a photoresist mask so as to partially
remove the AlSi film while leaving the prescribed portions covering
the contact holes CH1, CH2, CH3, and CH4, thus forming the pad
conductive films 180 composed of AlSi. At this time, the AlSi film
is subdivided into the prescribed areas in connection with the
contact holes CH1, CH2, CH3, and CH4 while forming the side
surfaces defining the outlines of the pad conductive films 180. The
patterning of the AlSi film can be achieved by way of wet etching
by use of a spin processor using an etchant (e.g. a mixed acid of
phosphoric acid, nitric acid, and water) in prescribed conditions
at a heating temperature ranging from 60.degree. C. to 75.degree.
C. (preferably, 65.degree. C.), processing time ranging from 30 sec
to 120 sec (preferably, 60 sec), and rotation speed ranging from
600 rpm to 1,000 rpm (preferably, 800 rpm). Due to wet etching, the
side surfaces of the pad conductive films 180 are activated and
exposed so that they are easily corroded. On the other hand, in dry
etching, the side surfaces of the pad conductive films 180 are
exposed to chlorine gas and are processed by etching, so that they
are easily corroded.
To cope with the above drawback, in an eighth step of the
manufacturing method shown in FIG. 37, a deposited film composed of
silicon nitride is formed on the entire surface so as to protect
the side surfaces of the pad conductive films 180 by way of plasma
CVD. Specifically, the silicon nitride film is formed with the
thickness of 1.6 .mu.m by way of low-stress plasma CVD in
prescribed conditions at a heating temperature of 400.degree. C.,
pressure of 2.5 Torr, SiH.sub.4 flow of 0.3 SLM, NH.sub.3 flow of
1.75 SLM, and bias power (RF H/F) of 0.44 kW/0.351 kW.
In a ninth step of the manufacturing method shown in FIG. 38 as
well as FIG. 13B, the silicon nitride film is partially removed by
way of dry etching using a photoresist mask while leaving the
surfaces of the pad conductive films 180 (formed in the contact
holes CH1, CH2, CH3, and CH4) except for center portions and the
surrounding areas of the pad conductive films 180 on the surface
insulating film 170. As a result, the pad protective films 190 are
formed using the silicon nitride film having superior protection
property for protecting the side surfaces of the pad conductive
films 180. The patterning of the pad protective films 190 is
achieved by way of dry etching using a parallel-plate plasma etcher
in prescribed conditions at CF.sub.4+O.sub.2 mixed gas flow of 150
SCCM, pressure ranging from 0.8 Torr to 1.2 Torr (preferably, 1.0
Torr), bias power of 250 W, and annealing temperature of 80.degree.
C. for 130 sec. After annealing of the silicon nitride film which
is formed with the thickness of 1.6 .mu.m by way of low-stress
plasma CVD, stress ranging from 100 MPa to 1 GPa may remain in the
silicon nitride film. In the second embodiment, the patterning is
performed in such a way that the pad protective films 190 are
locally formed in only the surrounding areas of the pad conductive
films 180; hence, it is possible to suppress the distortion of the
sensor chip 10 due to relatively high stress remaining in the pad
protective films 190.
In a tenth step of the manufacturing method shown in FIG. 39,
anisotropic etching is performed using a photoresist mask so as to
form a plurality of through-holes 170a (corresponding to the plate
holes 162c) in the surface insulating film 170, thus forming the
plate holes 162c in the upper conductive film 160. This step is
consecutively performed using the surface insulating film 170
having the through-holes 170a as a mask for use in etching of the
upper conductive film 160.
In an eleventh step of the manufacturing method shown in FIG. 40,
the plating protective film 200 composed of silicon oxide is formed
on the surface insulating film 170 and the pad protective films
190. At this time, the plating protective film 200 is embedded in
the plate holes 162c and the through-holes 170a of the surface
insulating film 170. The plating protective film 200 is subjected
to patterning by way of etching using a photoresist mask, thus
exposing the center portions of the surfaces of the pad conductive
films 180 covering the contact holes CH1, CH2, CH3, and CH4.
In a twelfth step of the manufacturing method shown in FIG. 41, the
bump films 210 composed of Au are formed on the surfaces of the pad
conductive films 180, which are formed in the contact holes CH1,
CH2, CH3, and CH4 and are exposed in the through-holes of the
plating protective film 200, by way of electroless plating. In
addition, the backside of the substrate 100 is polished so as to
achieve a desired thickness of the substrate 100 for use in an
actual product.
In a thirteenth step of the manufacturing method shown in FIG. 42,
etching is performed using a photoresist mask so as to form the
through-hole H5 for exposing the etching stopper ring 161 in the
plating protective film 200 and the surface insulating film 170.
This substantially completes the processing on the surface of the
substrate 100.
In a fourteenth step of the manufacturing method shown in FIG. 43,
the photoresist mask R1 having the through-hole H6 is formed on the
backside of the substrate 100 in order to form a through-hole
corresponding to the back cavity C1 in the substrate 100.
In a fifteenth step of the manufacturing method shown in FIG. 44,
the through-hole of the substrate 100 is formed by way of Deep-RIE.
At this time, the lower insulating film 110 serves as an etching
stopper.
In a sixteenth step of the manufacturing method shown in FIG. 45,
the photoresist mask R1 is removed from the backside of the
substrate 100; then, unnecessary deposits attached to an interior
wall 100c of the through-hole which is roughly formed in the
substrate 100 by way of Deep-RIE.
In a seventeenth step of the manufacturing method shown in FIG. 46,
isotropic etching is performed using the photoresist mask R2 and
buffered hydrofluoric acid (BHF) so as to remove the plating
protective film 200 and the surface insulating film 170 from above
the plate 162 and the plate lead 162d. In addition, a part of the
upper insulating film 130 is removed so as to form the ring-shaped
portion 132, the plate spacers 131, and the gap layer C3.
Furthermore, a part of the lower insulating film 110 is removed so
as to form the guard spacers 103, the diaphragm spacers 102, the
ring-shaped portion 101, and the gap layer C2. At this time, an
etchant (i.e. BHF) enters into the through-hole H6 of the
photoresist mask R2 and the opening 100a of the substrate 100. The
outline of the upper insulating film 130 is defined by the plate
162 and the plate lead 162d. That is, the ring-shaped portion 132
and the plate spacers 131 are formed by way of self-alignment of
the plate 162 and the plate lead 162d.
As shown in FIG. 47, undercuts are formed in the edges of the
ring-shaped portion 132 and the plate spacers 131 due to
anisotropic etching. The outline of the lower insulating film 110
is defined by the opening 100a of the substrate 100, the diaphragm
123, the diaphragm lead 123d, the guard electrodes 125a, the guard
connectors 125b, and the guard ring 125c, wherein the guard spacers
103 and the diaphragm spacers 102 are formed by way of
self-alignment. As shown in FIGS. 47 and 48, undercuts are formed
on the edges of the guard spacers 103 and the plate spacers 131 due
to anisotropic etching. In this process, the guard spacers 103 and
the plate spacers 131 are formed together with the spacers 129 for
supporting the plate 162 above the substrate 100 except for the
guard electrodes 125a.
Thereafter, the photoresist mask R2 is removed; then, the substrate
100 is subjected to dicing, thus completing the production of the
sensor chip 10 for use in a condenser microphone. The sensor chip
10 and the circuit chip are bonded onto a package substrate (not
shown); the terminals 125e, 162e, 123e, and 100b of the sensor chip
10 are electrically connected to the terminals of the circuit chip
(not shown) by way of wire bonding; then, the package substrate is
covered with a package cover (not shown), thus completing the
production of the condenser microphone. When the sensor chip is
attached to the package substrate, the opening of the back cavity
C1 at the backside of the substrate 100 is closed in an airtight
manner.
(4) Variations
The second embodiment can be modified in a variety of ways. For
example, it is possible to produce the condenser microphone having
a single-chip structure by incorporating the circuit elements such
as the charge pump CP and the amplifier A (which is originally
installed in the circuit chip) into the sensor chip 10, which has
at least one chip corresponding to the terminals 125e, 162e, 123e,
and 100b. It is preferable that the pad protective films 190 be
formed in narrow areas as long as they cover the side surfaces of
the pad conductive films 180 (serving as electrode films), wherein
the pad protective films 190 can be formed in any shapes such as
circular shapes, polygonal shapes, and ring shapes, and wherein the
pad protective films 190 can be integrally formed and unified with
respect to the combination of the terminals 123e and 100b and the
combination of the terminals 125e and 162e respectively.
Alternatively, it is possible to concentrate the alignment of the
terminals 125e, 162e, 123e, and 100b in a very small area so that
the pad protective films 190 therefor are integrally united
together.
In the first and second embodiments, the materials and dimensions
are merely illustrative and not restrictive, wherein the addition
and deletion of processes and the change of the order of processes,
which those skilled in the art can easily anticipate, are omitted
in the descriptions. In manufacturing processes, for example, the
compositions of films, the film formation methods, the methods for
defining outlines of films, and the order of processes can be
appropriately selected in response to the combination of film
materials whose properties satisfy the requirements of condenser
microphones, the thicknesses of films, and the required precisions
of defining outlines of films.
Moreover, the present invention can be applied to any types of
electronic devices and sensors except condenser microphones, such
as ultrasonic sensors, vibration transducers, pressure sensors, and
acceleration sensors.
Lastly, the present invention is not necessarily limited to the
above embodiments and variations, which can be further modified in
a variety of ways within the scope of the invention as defined in
the appended claims.
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